The overall hypothesis of this proposal is that availability of Fe and/or protein deficiency limit RBC formation in preterm infants and that optimization of Fe availability or of protein intake will result in clinically relevant enhancement of erythropoiesis and amelioration of anemia. Three specific Aims will be pursued in clinical studies and one Specific Aim in animal studies. Aim #1 will test the hypothesis that, by raising oral Fe intake from the current routine level to a high level, enhancement of erythropoiesis and improvement of Fe nutritional status will be achieved, without incurring toxic effect. To obtain quantitative estimates of Fe absorption and utilization at both levels of Fe intake, absorption and RBC incorporation of Fe will be determined with the use of the non-radioactive isotope, 58Fe in a subset of infants. In another subset, the PI will examine the effect of RBC transfusion on Fe absorption and/or excretion. Evidence of free radical damage will be obtained at frequent intervals. Aim #2 will test the hypothesis that provision of protein intakes that match estimated requirements more closely than current routine feedings will enhance erythropoiesis, besides leading to improved growth. Experimental infants will receive a supplement of bovine whey protein hydrolysate, whereas control infants will receive equicaloric amounts of lactose. Aim # 3 will test the hypothesis that treatment with r--HuEPO and Fe leads to a shortening of RBC life span and/or to an expansion of RBC mass or blood volume. This study intends to clarify observations that such adverse effects of R-HuEPO and Fe treatment are in fact, occurring. Because parenteral administration of Fe may substantial advantages over enteral administration, but it is potentially more offer toxic, newborn anemic lambs will be used in Aim #4 to test the hypothesis that intravenous Fe, when given alone or in combination with R-HuEPO, stimulates erythropoiesis. These experiments will also test the hypothesis that intravenous Fe can be given safely in doses that stimulate erythropoiesis. Aim #4, if successful, will provide the basis for future clinical trials of parenterally administered Fe in human infants.